1. Field of the Invention
The present invention relates to a method for producing an engine valve filled inside with metallic sodium.
2. Description of the Related Art
In an engine valve of an automobile or the like, in order to respond to an increase in performance, a reduction in fuel consumption of an engine, and the like, the weight is reduced and the heat transmission is increased by providing a hollow portion in the engine valve and filling the hollow portion with metallic sodium.
Such an engine valve is produced as follows. First, for example, in a solid round-bar shaped material, an umbrella portion is formed by forging or the like and a hollow portion is formed with a punch and the like in an inner part of a portion that becomes a stem portion. Thereafter, the stem portion is inserted into multiple tubular dies different in size sequentially and is drawn sequentially in such a way that sizes of an outer diameter and an inner diameter of the stem portion are reduced stepwise and the length of the stem portion is increased stepwise. The stem portion having an intended size is thus formed (for example, see Patent Literature 1 listed below).
Next, (1) solid metallic sodium is extruded into a bar shape, cut into a predetermined length, and then inserted into the hollow portion; or (2) solid metallic sodium is heated to melt (about 120° C. to 300° C.) into a liquid state and then put into the hollow portion by injecting a predetermined amount of metallic sodium into the hollow portion. Thereafter, an opening at an upper end of the stem portion is sealed (for example, see Patent Literature 2 listed below). The engine valve filled inside with metallic sodium is thus produced.